A nonwoven fabric is defined as an interlocking polymer network characterized by flexibility, porosity and integrity. The individual fibers used to compose the nonwoven fabric may be synthetic, naturally occurring, or a combination of the two. The individual fibers may be mechanically, chemically, or thermally bonded to each other. Nonwovens are used commercially for a variety of applications including disposable articles such as household wipes, surgical drapes, medical dressings, diapers, adult incontinent products and sanitary napkins. Tissue paper is a material closely related to nonwoven fabric in which the individual fibers may or may not be chemically bonded to one another. As used herein, nonwoven fabric and nonwoven may be used interchangeably and are intended to include such tissue paper.
In the aforementioned applications it is necessary to adhere the nonwoven substrate to a second substrate. The second substrate may be another nonwoven of a natural or synthetic polymeric substrate, such as a cellulosic, polyester or polyolefin. A commonly employed technique to bond the substrates together is the use of a water-borne, redispersible laminating adhesive. Suitable laminating adhesives must possess adequate adhesion to the substrates involved. For nonwoven applications they must also possess good flexibility, no staining or bleed through, suitable viscosity, and bonding range in order to function on commercially available equipment.
A variety of nonwoven applications have been developed which require that the water-borne laminating adhesive demonstrate appreciable water solubility, redispersibility or sensitivity. In these situations the water-borne laminating adhesive must provide a durable bond between the nonwoven substrate and the second substrate until exposed to a predetermined condition (e.g., water), after which the adhesive would release from the substrate(s). This water releasability is a particularly desirable property in the disposable market where flushability and/or degradeability are becoming critical.
Heretofore, the adhesive of choice in such disposable applications comprised homopolymers and copolymers of vinyl acetate prepared in the presence of hydrolyzed polyvinyl alcohol. However, such polymers require the formulation of additional polyvinyl alcohol into the adhesive compositions to impart the desired water sensitivity/redispersibility to the adhesive. Furthermore, the inclusion of significant amounts of plasticizer in adhesive formulations containing such polymers tends to decrease the water redispersibility of the adhesive, especially at elevated temperatures. In applications requiring such plasticizers, this presents problems and limitations in formulating adhesives which possess both required flexibility and water redispersibility. It would be desirable, then, to develop a water-borne laminating adhesive for use in such nonwoven applications which exhibits both improved adhesion and water redispersibility over adhesives known heretofore, while at the same time providing the formulator of the adhesive with the added flexibility of utilizing plasticizers where required.